1. Field of the Invention
The invention relates to thermosetting resinous coating compositions, particularly to elastomeric coating compositions.
2. Brief Description of the Prior Art
Elastomeric coating compositions are those which have excellent flexibility, particularly at low temperature, along with good hardness and durability. Such coating compositions are particularly useful on elastomeric parts of automobiles and trucks such as bumpers, front end hood sections and the side moldings.
Examples of elastomeric coating compositions are shown in U.S. Pat. No. 3,962,522. These coating compositions are based on polyesterurethane polyols cured with aminoplast. Another example of an elastomeric coating composition is that disclosed in U.S. Pat. No. 4,154,891. These coating compositions are based on polymeric polyols such as soft polyurethane polyols cured with a stoichiometric excess of an aminoplast curing agent.
The coating compositions of the aforementioned prior art are not sprayable at high resin solids content. This is a disadvantage in that multiple coating applications are required to get the desired coating thickness and appearance. The coating compositions of the present invention, although requiring multiple coatings, do not require as many as with the aforementioned prior art coating compositions. Also, future air pollution standards will require a reduction of organic solvent emissions of today's organic solvent-based coating compositions. Therefore, the coating compositions described in the aforementioned prior art may not be environmentally acceptable in the future.
Also, the coating compositions of the present invention are resistant to "solvent popping". Elastomeric coatings are typically applied in multiple coats with a solvent flashing step between coats. If the coatings are applied at too heavy a deposit, insufficient solvent will be removed in the flashing step. This results in excess solvent being entrained in the uncured coating. Although the solvent will be released during the curing step, the surface of the coating is undergoing a crosslinking reaction and the solvent must "burst" through the crosslinking film surface. This results in a permanent hole in the surface of the coating and the phenomena is referred to as "solvent popping". The coating compositions of the present invention have less of a tendency to solvent pop than coating compositions in the aforementioned prior art.
U.K. Pat. No. 1,477,008 discloses coating compositions comprising thermosetting resins, for example, hard polyester polyols, soft polyurethane polyols and aminoplast curing agents. These coating compositions, however, do not have the elastomeric properties of the coating compositions of the present invention.